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Artificial Atom: What and Why

Artificial atoms, also called quantum dots, are actually made of dozens of atoms, but they act very much just like a single atom in one important respect: when you provide them with the right amount (quanta) of energy, they will emit light with a very distinct color. This property makes them useful in applications as diverse as data communications and medical testing.

To understand how an artificial atom emits light, first consider how a real atom does it. In a real atom, electrons surround a dense nucleus and occupy different “energy levels.” When a particle of light called a photon strikes the atom, an electron will take that energy to temporarily “leap” to a higher energy level. When that “excited” electron comes back “down,” it will release that energy to emit a new photon with a particular frequency, or color. Each different kind of atom (i.e. each element in the periodic table) emits a unique combination of light colors because of the limited number of ways its electrons can change or leap from one state to another. The rules of physics at the size scale of atoms, called quantum mechanics, are what account for these rules or limitations.

When we combine dozens of atoms to make a quantum dot, it is still small enough for these quantum rules to hold. In my lab, we typically make quantum dots out of semiconducting elements, such as silicon or even molecules such as indium arsenide. When we illuminate a quantum dot with the right amount of energy, just like an atom it will absorb that energy and then emit photons of only particular wavelengths. The wavelengths are determined by the size of the dot, which depends on the number of atoms that compose the dot. By controlling the number of atoms in the dots, we are producing an “artificial periodic table,” in which the energy levels of the atoms determines how the electrons are arranged and therefore how they can leap between levels in our artificial atom.

So what good are artificial atoms? In my research we use them to create lasers or to amplify light. We can make quantum dots that emit desired colors of light when they are exposed to electrical currents in a process is similar to that at play in an LED. The current is a flow of negatively charged electrons going one way and holes (gaps in a material where electrons should be, but aren’t) flowing the opposite direction. When the electrons meet the holes, they give off a photon. Properly added to a material, quantum dots can make this happen efficiently, because they can concentrate the flow of electrons and holes much like the drain in a shower. We’ve been able to make artificial atom lasers and amplifiers to improve how we use light in data communications and computer chips.

We still have a lot to learn about quantum dots, the key challenges today are making them precisely the size we want and placing them exactly where we want them in electronic and photonic devices. We’d like better control of where they end up.

Despite the challenges, quantum dots are proving useful as engineering research tools and are making their way into commercial use. Their structure is artificial, but their value is real.

Prof. James Harris, Stanford University

History and Development of Semiconductor Lasers

Extensive research in arsenide and phosphide based III-V semiconductors have lead to the maturity of today’s electronics and optoelectronics technologies. However, these materials are limited to medium and narrow bandgap applications, preventing the fabrication of wide bandgap devices such as high power electronics and short wavelength optoelectronics. The development of wide gap material has been slow and problematic.

In the late eighties, interest in wide gap materials for short wavelength electronics and optoelectronics started to grow. Most research is concentrated on zinc selenide (ZnSe), silicon carbide (SiC) and some works on nitrides. SiC was used to produce the first commercial blue LEDs. However, lasers cannot be realized by this material due to indirect bandgap. Instead, ZnSe-based materials were pursued, leading to the production of first pulse blue laser diode in 1991. Further developments using ZnSe were problematic because wide gap in ZnSe is due to its highly ionic bonds that are weak. Conversely, SiC and nitrides have wide gaps due to strong chemical bonds. As a result, defect growth and propagation are common in ZnSe, leading to device degradation. In addition, ZnSe suffers from poor electrical/thermal properties and Fermi level pinning.

Nitride research started since early seventies when many physical properties such as refractive index, bandgap and lattice constant were measured. However, interest soon diminished, as it was discovered that no suitable substrate materials were available and the growth of p-type material was not possible due to strong background p-type doping. Interest in nitrides rekindled in early nineties when a small research group led by Nakamura at a virtually unheard chemical company reported blue emission from InGaN devices. The problems associated with the substrate mismatch were overcome using buffer layers and background doping was reduced via optimized growth. The p-doping was finally feasible by using magnesium dopant activated by thermal annealing. Thus, nitride based p-n junction could be produced and subsequently advance heterostructures LEDs and lasers.

The key breakthrough towards bridging the gap between arsenide (medium and narrow band gap) and nitrides (wide bandgap) was made by Weyers et al in 1992. Weyers et al made an unexpected discovery of a rapid reduction in bandgap energy with increasing nitrogen. This is in contrast with the general rules of III-V alloy semiconductors where smaller lattice constant increases the bandgap. The large electronegativity of N and its small covalent radius cause a very strong negative bowing parameter and the addition of N to GaAs or GaInAs dramatically decreases the bandgap. This behavior means that dilute nitride (alloy with low percentage of N composition) cannot be used as light emitters but huge potential in narrow gap application were clear incentives to pursue these materials. Surprisingly, the next few years saw very little published work on dilute nitrides.

Interest in dilute nitrides really began in the mid nineties after Kondow et al published a result on the quaternary alloy, GaInNAs. This new alloy allowed independent control over the In:Ga and N:As ratios. Increasing the In:Ga ratio will reduce the bandgap energy and increase the lattice constant while increasing the N:As ratio also causes bandgap reduction but decrease the lattice constant. Such tailoring potential opens up a wide range of possible applications, however, the 1.3 micrometer laser based on GaAs for optical communications were identified as a key application. At this wavelength, silica fibre has zero dispersion and relatively low attenuation, making it an attractive communication window.

At present, almost all of the long wavelength communication lasers in use are produced using InGaAsP/InP. This is because, for many years it was believed that there was no suitable alloy lattice matched to GaAs that would emit >1.1micrometer except InGaAsP (until Kondow report on GaInNAs). However, this material is not ideal for producing cheap lasers as a number of problems ultimately increase costs.

At present, the main research drive for alternative 1.3 micrometer lasers are split between self-organised quantum dots (QD) and dilute nitrides. QD lasers have attracted substantial interest, as the physics of QDs could potentially improve laser performance considerably. They are expected to have reduced temperature dependence, reduce thresholds and higher efficiencies. However, this can only be realized if methods are found to control the dot density, distribution and most importantly size. However, the expected temperature performance of QD lasers has not yet been demonstrated.

On the other hand, progress in dilute nitride has been rapid. This is because GaInNAs/GaAs lasers has good temperature performance due to deep electrons confinement. In addition, being based on GaAs, dilute nitrides can be easily integrated with the established GaAs technology, including AlGaAs based Bragg mirror. The emission wavelength has even been pushed to 1.55 micrometer using GaInNAs with high indium and nitrogen fractions.

Reference:

http://wave.prohosting.com/rjpott/dilutenitrides.html

J S Harris, "GaInNAs long wavelength lasers: progress and challenges," Semicond. Sci. Technol. 17 (2002), 880-891